As an energy storage system, a battery and a system thereof are means for storing electric energy so as to restore the energy when needed. Typically, a cell includes two electrodes arranged in the electrolyte, i.e. an anode and a cathode. As known in related technologies, an electrical apparatus to be operated is usually connected across the cathode and anode at two ends to obtain electrical energy from the cell.
Invented in 1859, the lead acid battery has more than 150 years of history. It still remains to be one of the most popular batteries nowadays and has been widely used in technical fields such as electricity, communication, railway, petroleum, aviation, irrigation, coal, geology, medical care, rail transportation, national defense facilities and so on.
A lead-acid battery as well as a system thereof is such a device that converts electrical energy into chemical energy for storage and then converts the chemical energy into electrical energy to be supplied to electrical apparatuses for use when needed. The cathode active substance in the lead-acid cell is PbO2, and the anode active substance is sponge-like lead (Pb), and the electrolyte is liquid H2SO4. The process of charging and discharging the lead-acid cell is achieved by electrochemical reactions. As shown in the following reaction equation, Pb (anode) and lead oxide (cathode) react with H2SO4 during the discharging process of the lead-acid cell to generate lead sulfate. The charging process exhibits a reverse reaction of the discharging process.
Cathode reaction: PbO2+4H++SO42−+2e−PbSO4+2H2O
Anode reaction: Pb+SO42−PbSO4+2e−
Overall reaction: Pbo2+Pb+2H2SO42PbSO4+2H2O
Currently, the lead acid battery is widely used all over the world with more than 50% of battery market share due to its reliability and low cost. Traditional lead-acid batteries are mostly used in small-scale and low-rate applications, such as those in auxiliary devices or back-up power, so the overheat and heat dissipation problems are not so noticeable, nor are there any particular solutions to these problems. However, with recent development of smart grid and the increasing amount of interest in renewable energy (e.g. wind power, solar energy and so on), the demand in large-scale energy storage system has never been more imperative. At present, lead-acid batteries have already been used in Uninterruptible Power Supplies (UPS). The emerging applications in technology pose new challenges to lead-acid batteries and other types of batteries. For example, overheating occurs in almost any large-scale applications that are usually of high rate and multi-cycle, which shortens the cycle life and sometimes even causes permanent damage to the batteries and especially to the lead acid batteries. Therefore, the thermal management technology is extremely important for the lead acid battery in large-scale applications in maintaining proper system health etc.
The conventional solution for overheating is to over-size the batteries, so that the relative discharge rate and the depth of discharge are smaller. But this, on the other hand, increases the total system cost tremendously.
The prior art lead-acid batteries are mainly classified into two types: flooded type and valve-regulated type. The heat dissipation problem is more severe in the valve-regulated type lead-acid batteries (VRLA batteries) than the flooded type lead-acid batteries because excess electrolyte in cells of the latter fills the three-dimensional space in the cells except for those occupied by the electrodes, thereby the thermal contact between internal members of the cells is enhanced. Gases are generated during charging and the gases remove heat from the cell via water loss and acid mist. In contrast, in a cell of a valve-regulated type lead-acid battery, the acid liquid is absorbed in saturation by a separator (e.g. absorptive glass fiber fabric), so there is no excess liquid electrolyte in the cell. The limited contact of the acid, separators and plates with the plastic case walls limits the heat transfer out of the cell because of the lack of a heat dissipation passage and therefore increases the operating temperature, which limits the cycle life of valve-regulated type lead-acid battery and thus their potential applications in large-scale.
Overheating of the valve-regulated type lead-acid battery, as a matter of fact, is mainly caused by heat release from chemical reactions and ohmic heat (resistance heat generated from grid plate, bus-bar, separator, terminal post and etc. of the cell due to their resistance). The chemical reaction is very intense, for example the oxygen recombination reaction during the charging process is exothermic with an enthalpy of 68.32 kcal/mol. When the temperature of the positive plate is increased, the rate of oxygen evolution increases rapidly and a bigger portion of oxygen recombines at the negative plate, giving rise of a further temperature rise there. The cell temperature can easily exceed 80° C. and the cell can go into ‘thermal runaway’ thus forcing the cell temperature even higher. In some instances thermal runaway can lead to softening or even burning of the polymer case. The ohmic heat (I2R) also comes from the inside of the cell. The shell of a cell is generally made of polymer materials, and the heat dissipation contact area is very limited through metallic components of a cell such as grid plate, bus-bar and terminal post, thus heat within the interior of the cell is not easy to be dissipated.
In high rate applications of a battery, according to Arrhenius Equation, reaction rate of any chemical reaction is generally increased to as much as two times with the increase of temperature by 10 degrees. This principle is applicable to product life approximation based on failure mode (chemical reaction, such as corrosion, oxygen recombination reaction and so on), in particular applicable to the life approximation of a lead-acid battery. According to IEEE Recommended practice for Maintenance, Testing and Replacement of Vented Lead-acid Batteries for Stationary Applications, IEEE power engineering society, IEEE std 450™-2002, 3 Apr. 2003, it is calculated that the life of a lead acid battery is shortened by 50% when the working temperature of the lead acid battery increases from 25 to 33.
In order to prevent overheating of a battery and its system and prolong life thereof, various solutions have been proposed at present for thermal control or thermal management during operation of the battery, wherein most of the solutions focus on thermal control or thermal management on a side or bottom of a cell, e.g. U.S. Pat. No. 7,967,256, U.S. Pat. No. 7,531,270, U.S. Pat. No. 6,533,031, U.S. Pat. No. 6,512,347, U.S. Pat. No. 6,407,553, U.S. Pat. No. 5,695,891, U.S. Pat. No. 5,356,735, U.S. Pat. No. 5,385,793, U.S. Pat. No. 4,913,985. These modified designs relate to built-in arrangements which are also technically challenging in maintenance and heat dissipation. As stated above, heat generated in a valve-regulated type lead-acid battery is difficult to be dissipated to the outside, so heat dissipation effect is not quite satisfactory when the prior art thermal control or thermal management method is applied to the valve-regulated type lead-acid battery.
U.S. Pat. No. 7,651,811 discloses a traction battery, comprising a ventilated plastic cover for covering an electrical connection strap, wherein a fan forces air to flow through the electrical connection strap of the battery to reduce the operating temperature of the battery. U.S. Pat. No. 3,834,945 discloses to use water to cool terminal posts and inter-cell electrical connection strap for a traction battery. No matter cooling is performed by air or water, improvement in heat exchange is not satisfactory due to the limited heat-exchanging area of the electrical connection strap. In addition, the design of a structure having a function of cooling the battery, e.g. the added water cooling system or a fan, etc., makes the entire structure of the battery more complex and the volume thereof large and heavy, which results in a complicated maintenance and installation process.
CN200952916Y discloses an improved structure of a heat radiation device for improving heat radiation efficiency of a mechanical apparatus which generates heat during operation, wherein a nanometer carbon layer is attached in a physical manner to the outside of the heat radiating fin of the machine, so that the heat radiating effect of the heat radiation device is enhanced by increasing the whole heat radiating area and by rapid radiation characteristic of the nanometer carbon layer. This document, however, does not disclose any effective composition of the nanometer carbon layer, nor teach or suggest application of the same to other technical fields. No reports have been found so far to successfully solve the overheating problem of an energy storage system by applying a heat dissipation coating, in particular a heat dissipation coating of high emissivity to an energy storage system, e.g. a valve-regulated type lead-acid battery and its system.
The above documents are incorporated herein in entirety by reference.
Therefore, the present invention is aimed to improve one or more defects in the prior art.